In recent years, a technology of making a plurality of vehicles run in a row with a short vehicle-to-vehicle distance has gained attention in order to improve a traffic flow and improve fuel efficiency with a reduction of air resistance. Hitherto, there has been known a method of controlling a vehicle-to-vehicle distance between a front vehicle and each of subsequent vehicles of a row. In this method, there is a problem in that a vehicle-to-vehicle distance error is propagated to the rear vehicle when the foremost vehicle is disturbed by a slope of a road, a wind, or the like. In order to solve this problem, a system is proposed in JP-A-10-162282. In this system, the row-running of the plurality of vehicles is realized by controlling the vehicle-to-vehicle distance between the foremost vehicle and each of the subsequent vehicles in the row. According to this method, it is considered that the vehicle-to-vehicle distance error is not propagated to the rear vehicle.